1. The Field of the Invention
The present invention is related to image projection systems. More particularly, the present invention is related to an off-axis image projection display system that produces a high contrast ratio and degree of brightness which enhances the quality of a projected image.
2. The Relevant Technology
Traditional projection displays include various components such as a light source, illumination optics, projection optics, spatial light modulators for image formation, and a color splitting assembly. Generally, projection displays manipulate a light beam from the light source and separate the light beam into three color channels, each comprising one of the three primary colors, red, green, and blue. Each color channel is manipulated by the spatial light modulators to form the desired portion of the final image. The portions of the image formed by each color channel are then recombined to create the desired image which is projected upon a viewing screen.
Each component in the projection display has particular characteristics that are important to the operation of the projection display. For example, spatial light modulators or reflective imagers are necessary to generate the requisite image to be projected onto a screen by way of the projection optics. The reflective imagers can take various forms, such as liquid crystal light valves (LCLV""s ), and more specifically, by way of example and not limitation, liquid crystal on silicon (LCOS) imagers. An LCLV includes a number of polarizing elements that manipulate light incident thereupon, such that the light reflected from the polarizing elements has different polarization states, which is dependent on the image to be projected. As such, LCLV""s include drive circuitry to influence the polarizing element and therefore create the requisite image to be projected onto the viewing screen through use of the projection optics.
More specifically, the individual pixels of the LCLV are activated to an xe2x80x9conxe2x80x9d state by the application of a voltage or signal to individual pixels, and the polarization of the light reflected from those pixels is rotated by 90xc2x0 with respect to the light incident upon the LCLV. In the xe2x80x9coffxe2x80x9d state, the light which is reflected from the pixels has no rotation of the polarization state. The light reflected, whether from xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d state pixels, is recombined within the color splitter and redirected to the polarizing element. The polarizing element then acts as an analyzer and transmits light which has undergone a reversal of polarization state at the LCLV into the image projection optics to be projected upon a viewing surface, such as a screen. Since light which is not rotated in polarization by the LCLV is not transmitted to the image projection optics, the final image is formed from the selected pixels of the three imagers and includes the three primary colors. The dark spots on the image correspond to the location of the pixels in the xe2x80x9coffxe2x80x9d state or where the light is not transmitted by the polarizing element.
A major aspect of image quality in a projection display system is measured through a parameter known as the contrast ratio, which is defined as the ratio of the light transmitted through the system in the xe2x80x9conxe2x80x9d state divided by the amount of light transmitted in the xe2x80x9coffxe2x80x9d state. The higher the contrast ratio, the better the overall quality of the image. A display should project a bright image relative to the ambient lighting conditions. High brightness of the xe2x80x9conxe2x80x9d pixels enhances the contrast ratio and allows the projector to be used in a broader range of ambient lighting conditions, i.e., a darkened room is not required.
The properties and characteristics of the optical components of conventional projection display systems influence the light propagating through the system and reduce the contrast ratio. In convergent imaging systems utilizing color splitting assemblies, such as a Philips prism, a significant loss of contrast results from the interaction of the geometrical effect from skew rays that are partially depolarized as a result of reflection at the optical coatings, such as antireflection and dichroic coatings, and the total internal reflection (TIR) surfaces.
The contrast ratio is reduced by this depolarization state of the light from the operation of the polarizing beam splitter and color splitting assembly. As stated previous, the contrast ratio is the ratio of light transmitted through the system in the xe2x80x9conxe2x80x9d state divided by the amount of light transmitted in the xe2x80x9coffxe2x80x9d state. More specifically, the contrast ratio is the ratio of wanted, such asp-polarized, to unwanted, s-polarized, polarization states of the light transmitted through the system (where s-polarization refers to light which has its polarization vector perpendicular to the direction of propagation, while p-polarization refers to light which has its polarization vector lying in the plane of propagation). Variations between the pure s or p polarization state cause a resultant effect on the system contrast ratio.
Generally, the polarization of light is modified through birefringent materials, i.e. a material whose refractive index varies as a function of direction. Birefringent materials are commonly used to form wave compensators or retardation plates. Quarter waveplates effectively introduce a relative phase shift of 90xc2x0 in one of the polarization components of the incident beam as the light goes through the material one time, if the plate is oriented perpendicular to the optical axis (the axis of the direction of propagation of the beam). A quarter waveplate defines a thickness of a birefringent material wherein the difference in the refractive index between the fast and slow axes results in a phase retardance of a quarter wavelength when comparing two beams of polarized light having their respective polarization vectors parallel to the fast and slow axes of the waveplate.
Various prior projection display systems have attempted to correct and control the effects of phase shift and/or diattenuation induced by depolarization by polarizing beam splitters, color splitting assemblies, and birefringent material.
Conventional reflective imaging systems, such as those disclosed in U.S. Pat. No. 5,459,593 to Ootaki, U.S. Pat. No. 5,594,591 to Yamamoto et al., and U.S. Pat. No. 5,327,270 to Miyatake, transmit light through a polarizing element, such as a polarizing beam splitter, that transmits or reflects a polarized component of the light to a color splitting assembly or color splitter. Light passing through the color splitting assembly to be incident on one or more LCLV""s is affected by the antireflection and dichroic coatings formed within traditional color splitting assemblies. The use of conventional anti-reflection and dichroic coating designs in a color splitter changes the polarization state of light passing therethrough, thereby reducing image contrast and brightness. The phase change, or retardation, and intensity differences between polarization states transforms plane polarized light into elliptically polarized light. To correct for elliptical depolarization, Ootaki and Yamamoto define special optical coating designs, however, this is undesirable for practical manufacturing reasons, as the increased complexity of coating design, i.e., number of layers and total thickness and the control thereof, increases the manufacturing expenses.
In addition to the influence of the color splitting assembly, the polarizing beam splitter causes rotation of the polarization vector of plane polarized light. This, however may be some what accounted for through use of a quarter waveplate as described in Miyatake. The elliptical polarization, however, is not corrected by the quarter waveplate. The combination of rotation and ellipticity of the polarization vector is the major source of light leakage, which is transmitted as if the LCLV were in the xe2x80x9conxe2x80x9d state when the reflective LCLV is in the xe2x80x9coffxe2x80x9d state, decreasing the contrast ratio and brightness, thereby detracting from the image quality.
A projection display system which avoids the use of a polarizing beam splitter is disclosed in U.S. Pat. No. 4,969,730 to van den Brandt, the disclosure of which is herein incorporated by reference. In the van den Brandt system, the input illumination travels through a linear polarizing element that controls the light to be incident upon an angle-dependent beam splitting element or prism. The system is configured such that light is reflected from the angle-dependent beam splitting element based on the properties of total internal reflection. The exiting light from the beam splitting element is not parallel or coincident with respect to the light reflected from the reflective imager, but is xe2x80x9coff-axisxe2x80x9d by a small angle. The light internally reflected is incident upon LCLV""s that modulate the polarization state in accord with the imager xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d state. The van den Brandt patent includes a second linear polarizer, termed an analyzer, located between the beam splitter and the viewing screen, that transmits an image representing the modulation of the image pixels by controlling the passage of a particular polarization state of light to the viewing screen. Both the polarizer and the analyzer absorb all polarization states that are not to be transmitted therethrough.
In use, both the polarizer and the analyzer of van den Brandt are bombarded with light beams containing numerous non-transmissive polarization states over both visible and non-visible wavelengths. The continual absorption of high intensity non-transmissive polarization states deteriorates the effectiveness of the polarizer and/or analyzer, resulting in the passage of multiple polarization states or degradation induced optical absorption. As such, the contrast ratio is reduced and the overall life of the projection display system is significantly diminished.
Although, the van den Brandt off-axis system avoids the use of a polarizing beam splitter, the contrast is still diminished by imperfect polarization of skew rays, and their additional depolarization in the color splitting element, which occurs on reflection or transmission by the various optical coatings.
It is a primary object of the present invention to provide apparatus and methods for improving the contrast ratio in an image projection display system.
It is another object of the present invention to provide apparatus and methods for separating incident light into three colors with minimum loss of intensity at the desired wavelengths to form a color image of high brightness, while simultaneously achieving high contrast.
It is yet a further object of the present invention to provide systems and apparatus for separating incident light into three colors which are efficient and cost effective to manufacture.
Still another object of the present invention is to provide an image projection display system that is compact, simple and relatively inexpensive and easy to manufacture, yet achieves high contrast levels.
Yet another object of the present invention is to provide an image projection display system which require substantially no phase control coatings to achieve high contrast and brightness levels.
Still a further object of the present invention is to provide an image projection display system that has a longer useful life due to minimization of deterioration of components used therein.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an image projection display system is provided that is substantially insensitive to the usually undesired depolarization effects of optical coatings or stress or thermal birefringence of optical components and assemblies. Through the use of an off-axis illumination configuration and reflective imagers each in optical communication with a respective polarizer, a high contrast ratio and degree of brightness is achieved which significantly improves the final image quality. The present invention is particularly useful in achieving enhanced contrast in reflective imaging systems such as those utilizing reflective liquid crystal on silicon (LCOS) imagers. The system of the invention minimizes or avoids the effects which arise from the rotations and ellipticity which occur in reflective imaging projection systems that reduce the contrast ratio by generating unwanted depolarization and contributing to light leakage.
In one embodiment, a beam splitter is configured to direct a first light beam along a first optical path toward at least one reflective imaging device adapted to direct a second light beam along a second optical path to produce an image to be projected. At least one polarizing device is interposed between the beam splitter and the imaging device, with the polarizing device configured to allow passage of a first polarized component of the light beam while blocking passage of a second polarized component of the light beam. In other embodiments, a color splitting assembly can be disposed along the first and second optical paths between the beam splitter and a plurality of reflective imaging devices and their respective polarizing filters to provide for color image production.